The use of a product stream of reformed gas as a source of heat in heat exchange reforming is known in the art. Thus, EP-A-0033128 and EP-A-0334540 deal with parallel arrangements, in which a hydrocarbon feed is introduced in parallel to a tubular reformer and heat exchange reformer. The partially reformed gas from the tubular reformer is then used as heat source for the reforming reactions in the heat exchange reformer.
Other parallel arrangements combine heat exchange reforming and autothermal reforming. EP-A-0983963, EP-A-1106570 and EP-A-0504471 deal with processes in which a hydrocarbon feed is introduced in parallel to a heat exchange reformer and autothermal reformer. The hot product synthesis gas from the autothermal reformer is used as heat exchanging medium for the reforming reactions occurring in the heat exchange reformer.
EP-A-0440258 discloses a process in which the hydrocarbon feed is first passed through a first heat exchange reformer to provide a partially reformed stream. The partially reformed stream is then introduced in parallel to a tubular reformer and second heat exchange reformer. The product streams from both reformers are combined and introduced to an autothermal reformer. The product gas from the autothermal reformer is used as heat source in the second heat exchange reformer, while the product gas from said second heat exchange reformer is used as heat source in the first heat exchange reformer.
Series arrangements are also known in the art. U.S. Pat. No. 4,824,658 and U.S. Pat. No. 6,296,679 (FIG. 2 in both references) disclose a process in which the entire hydrocarbon feed is first introduced to a heat exchange reformer, then passed to a tubular reformer and finally to an autothermal reformer. The product gas from the autothermal reformer is used as heat source in the heat exchange reformer.
U.S. Pat. No. 4,376,717 and our US 2009/0184293 disclose a process in which a hydrocarbon feed is first passed through a tubular reformer; the partially reformed gas is then subjected to heat exchange reforming and finally autothermal reforming. The product gas from the latter is used as heat source in the heat exchange reforming.
Our patent DK 148882 discloses a process for production of synthesis gas, in which the hydrocarbon feed is passed through a heat exchange reforming and autothermal reformer, and where the product gas from the latter is used as heat source in the heat exchange reformer.
The use of heat exchange reformers in the production of synthetic fuels by the Fischer-Tropsch synthesis has significant benefits. Potential benefits compared to other technologies include reduced oxygen consumption, lower capital cost per unit of product, and a higher carbon and energy efficiency. In plants for production of chemicals such as methanol, the use of heat exchange reformers has similar advantages.
In our co-pending patent application PCT/EP2011/006179 we disclose a process in which a hydrocarbon feed is passed through a heat exchange reformer and an autothermal reformer and where the product flow from the latter is used as heat source in the heat exchange reformer. Tail gas from the FT-synthesis is added after the heat exchange reformer and before the autothermal reformer. The addition of tail gas is adjusted to give the desired stoichiometric ratio of H2/CO equal to about 2 in the effluent stream from the autothermal reformer. Steam is added downstream the autothermal reformer to reduce the aggressivity of the gas passed to the heat exchange reformer with respect to metal dusting.
A specific embodiment also disclosed in PCT/EP2011/006179 is where part of the hydrocarbon feedstock bypasses the heat exchange reformer and is directed directly to the autothermal reformer. This reduces the efficiency of the total plant and increases the oxygen consumption compared to the embodiment without bypass.
It is generally preferred to direct all the product flow from the autothermal reformer to the heat exchange reformer. If only part of the flow is used as heating medium, the size of the heat exchange reformer must be increased to compensate for a lower driving force (lower temperature difference) for heat transfer.
Heat exchange reformers may also be coupled in a similar manner with other partial oxidation reactors. In addition to autothermal reforming (ATR), such reactors include non catalytic partial oxidation (POX) such as gasifiers, and catalytic partial oxidation (CPO), although autothermal reforming (ATR) is preferred. ATR and CPO are provided with a fixed bed of catalyst. As used herein the term autothermal reforming (ATR) encompasses also secondary reforming.
However, for many processes such as those mentioned above, especially for large scale plants comprising a heat exchange reformer and an autothermal reformer, it may be preferred to operate with a low steam-to-carbon ratio. This is for example the case when the produced synthesis gas is to be used for subsequent hydrocarbon synthesis via the Fischer-Tropsch (FT) synthesis. Operation at high steam-to-carbon ratios means higher flow rates due to the increased amount of steam in the feed. In other words, operating at high steam-to-carbon ratios means that the capital cost due to the use of larger equipment may be prohibitively large, especially for large scale plants. Furthermore, high steam-to-carbon ratio means that a larger amount of carbon dioxide is formed in the process. This is in many cases a disadvantage such as for example in plants for the production of synthetic fuels by the low temperature Fischer-Tropsch synthesis. In the low temperature Fischer-Tropsch synthesis carbon dioxide is considered an inert and not a reactant.
Operating at low steam-to-carbon ratios in plants comprising a heat exchange reformer creates a number of challenges. One such challenge is the risk of carbon formation on the catalyst in the heat exchange reformer:CH4→C+2H2  (1)
Carbon on the catalyst may also be formed from higher hydrocarbons (hydrocarbons with two or more carbon atoms in the molecule) or from carbon monoxide according to similar reactions as described in the literature.
The formed carbon may lead to catalyst deactivation and/or catalyst disintegration and build up of pressure drop. This is undesired.
The risk of carbon formation is linked to the catalyst temperature and the steam-to-carbon ratio. For a given feed gas composition and pressure, the steam-to-carbon ratio in the feed gas must be raised if the catalyst temperature is increased. As a higher steam-to-carbon ratio may be uneconomical as described above, it is important to be able to control the temperature in the catalyst of the heat exchange reformer to avoid addition of excessive steam. The temperature control thereby enables operation closer to the carbon limit without exceeding it. In most cases the catalyst temperature reaches its maximum at the outlet of the heat exchange reformer.
Another challenge for the use of heat exchange reformers is metal dusting corrosion. In many cases especially at low steam-to-carbon ratios the prevention or minimisation of metal dusting requires the use of high alloy and/or special materials in the reactor itself. Such alloys are generally expensive and it is therefore generally desired to minimise the size and the heat transfer surface of the heat exchange reformer reactor itself.
In the aforementioned processes comprising a heat exchange reformer upstream and in series with an autothermal reformer, the simultaneous control of the catalyst exit temperature from the heat exchange reformer and the synthesis gas production and quality (e.g obtaining the desired H2/CO-ratio of about 2 for production of synthesis gas for the FT-synthesis) is not straightforward. This is particularly the case when it is considered that the synthesis gas production unit is required to operate under different conditions such as part load and with different natural gas feed and tail gas compositions. Furthermore, the plant production should remain unaffected even in the case of progressive fouling of the heat exchange reformer itself. Fouling is known to the industry and in the art and has in this case the consequence that the heat transferred from the effluent of the autothermal reformer to the catalyst side of the heat exchange reformer is reduced.
In several of the processes described above, the hydrocarbon feed is passed through a heat exchange reformer and then an autothermal reformer and where the product flow from the latter is used as heat source in the heat exchange reformer.
Fouling is known to the person skilled in the art as unwanted material accumulating on the surface of the heat exchanging equipment. This material creates extra resistance to the heat exchange. In a heat exchange reformer the consequence will be that the heat transferred from the effluent of the autothermal reformer to the catalyst side of the heat exchange reformer is reduced.
In the initial period of operation, fouling is generally very low. However, over time, fouling of the heat exchange reformer surface may take place reducing the heat transferred from the autothermal reformer effluent stream to the catalyst side of the heat exchange reformer. This means the exit temperature from the catalyst side of the heat exchange reformer will drop if no countermeasures are taken. The autothermal reformer exit temperature will as a consequence of the lower heat exchange reformer exit temperature also decrease, which again will lead to an additional drop in the heat exchange reformer outlet temperature and so forth. This feed and effluent relationship between the autothermal reformer and heat exchange reformer means that the effect of fouling is amplified, thus a small decrease in the heat exchange reformer ability to transfer heat may lead to a large decrease in heat exchange reformer exit temperature.
This will in turn lead to reduced plant efficiency and/or reduced production, and/or increased oxygen consumption per unit of synthesis gas produced.
In order to maintain conversion of the hydrocarbon feedstock and maintain production, various countermeasures may be considered. The first is to preheat the feedstock to the heat exchange reformer to a higher temperature. However, this requires import of additional fuel and reduces the overall plant energy efficiency. Another alternative is to accept the lower exit temperature (and thereby the lower conversion of the hydrocarbon feedstock) from the heat exchange reformer and increase the conversion in the autothermal reformer. However, this requires additional oxygen and thereby increases the capital cost associated with the expensive air separation unit used to produce the oxygen. In addition, the plant efficiency will normally drop.
An alternative would be to design the size of the heat exchange reformer to take into account the reduction of the heat transfer due to fouling. However, in this case the temperature at the outlet of the heat exchange reformer on the catalyst side during the initial period of operation (before any significant fouling has occurred) could become too high increasing the risk of carbon formation on the catalyst.
As described above in one embodiment of the aforementioned patent application PCT/EP2011/006179, part of the hydrocarbon feedstock bypasses the heat exchange reformer and is directed to the autothermal reformer. In such an embodiment, the fraction of the hydrocarbon feedstock bypassing the heat exchange reformer can be used to control the temperature out of the heat exchange reformer. The heat exchange reformer may for example be designed for a specific bypass during start of operation (before fouling has taken place). As fouling occurs the fraction of the flow bypassing the heat exchange reformer can be increased to maintain the catalyst outlet temperature from the heat exchange reformer. Production can be maintained by increasing the hydrocarbon feedstock flow. However, the plant efficiency will also drop in this case.
It is thus the objective of the present invention to provide a process for production of synthesis gas comprising a heat exchange reformer and an autothermal reformer where the exit temperature from the heat exchange reformer and the synthesis gas production can be maintained during operation with no or very limited loss of efficiency and with no or a very limited increase in the oxygen consumption.